JP3709032B2 - Radar system - Google Patents

Description

[0001]
[Technical field to which the invention belongs]
The present invention relates generally to radar systems, and more particularly to an arrangement for nulling feedthroughs used in radar systems.
[0002]
[Prior art]
As is known in the art, in many applications, radar transmitters are required to be relatively small while operating at relatively high power levels. Furthermore, it is advantageous to use a solid state element in order to improve the reliability of the transmitter. One answer to these requirements is transmission using a continuous wave (CW) waveform that is distinct from a pulse wave.
[0003]
[Problems to be solved by the invention]
Using a CW waveform can minimize the size of the transmitter when implemented with solid state elements. This is because the design margin of the transmitter is constrained by the average power, not the peak power required for pulse waveform systems. However, the CW radar system will provide a high level of power directly from the transmitter to the receiver. This effect is commonly referred to as “feedthrough”. This feedthrough saturates the receiver and prevents proper detection of the desired reflected radar energy. That is, the receiver is close to the system's good thermal kTNF noise floor while maintaining simultaneous operation in a distortion-free manner even in the presence of relatively high feedthrough. It is required to provide sensitivity. Thus, in CW systems, the requirement for full sensitivity and high linearity becomes difficult to achieve as feedthrough levels increase.
[0004]
One technique that has been proposed to reduce feedthrough is to place the transmitter and receiver at different locations. This proposal is not sufficient when the location for both the transmitter and receiver is limited. Also, the antenna directivity pattern required to cover the intended area can be seen directly from the receiving antenna to the transmitting antenna, thereby making the receiver sensitivity due to the presence of high level feedthrough. It will be necessary to promote desensitization.
[0005]
More recently, it has been proposed to dispose the receiving antenna station at a position away from the radar receiver station and to couple the antenna station and the receiver station via an optical link.
[0006]
[Means for Solving the Problems]
The present invention provides a radar system having a radar transmitter and a radar receiver. The radar receiver includes an antenna station and a receiver station. The transmitter and the receiver station may be close to each other or separated from each other. However, the antenna station is located remotely (remotely) from at least one of the receiver station and the transmitter. Thus, here, the transmitter and / or receiver station is sometimes referred to as local and the antenna station is referred to as remote. A system for zeroing feedthrough is provided. The nulling system is located at a remote antenna station and includes a remote combiner that provides a feedthrough zero residual signal representative of the power difference between the signal received by the antenna station and the feedthrough nulling signal. A downlink modulator is located at the antenna station and produces an optical signal representing the feedthrough zero residual signal. A downlink optical detector is located at either the transmitter or receiver station and detects an optical signal representing the feedthrough zero residual signal. A variable attenuator and phase shifter located at either the transmitter or the receiver is provided with a portion of the signal transmitted by the transmitter. This variable attenuator and phase shift device is controlled by a signal derived from the detected optical signal representing the feedthrough zero residual signal and changes the gain and phase shift of a portion of the transmitted signal to change the feedthrough Produces a null signal. More specifically, the output of the variable attenuator and phase shifter is a replica of the transmitted signal with the appropriate gain and phase, thereby allowing local ups to the antenna station at either the transmitter or receiver station. After link optical transmission, an appropriate feedthrough nulling signal is generated for the remote combiner located at the antenna station.
[0007]
More particularly, the local uplink laser is remotely located from the antenna station and has the appropriate gain and phase to null the feedthrough signal received by the antenna station given the signal. An optical signal representing a replica of the transmitted signal is generated. The uplink detector is located at the antenna station and detects the optical signal produced by the uplink laser modulator to produce a feedthrough nulling signal.
[0008]
According to another feature of the invention, a local noise reduction system is provided. This noise reduction system consists of a local detector (ie, either a transmitter or a receiver station) that detects a local uplink laser modulator optical signal transmitted to and detected by an uplink detector located at the local antenna station. Detector). The local noise cancellation combiner is provided with a local detector (ie, a detector located at either the transmitter or receiver station) and a portion of the signal transmitted by the transmitter. The variable gain and phase shifter combines the output of the noise cancellation combiner with the detected optical signal representing the feedthrough zero residual signal and from the transmitter according to the detected optical signal and the output of the noise cancellation combiner. The signal gain and phase shift are changed to produce a replica of the transmitted signal with the appropriate gain and phase shift.
[0009]
In this configuration, the optical uplink signal generated by the local uplink laser modulator is divided into two independent detectors: a remote detector at the antenna station and a local detector at either the transmitter or receiver station. , Detected by. The local uplink laser noise is detected by a local detector located in close proximity and applied to the optical uplink signal in transmission to the local (and thus remotely located antenna station detector). The local and remote detectors are matched because they are tracked in a relatively high bandwidth noise reduction loop by a detector that is not exposed to noise. The local high bandwidth noise reduction loop acts to reduce uplink laser noise and linearly modulates this local uplink laser with a feedthrough zero signal. Thus, if the local and remote detectors are matched, noise is reduced at the local detector and at the same time at the remote detector.
[0010]
BEST MODE FOR CARRYING OUT THE INVENTION
Referring to FIG. 1, a radar system 10 having a radar transmitter 12 and a radar receiver 14 is shown. The radar receiver 14 includes an antenna station 16 and a receiver station 18. The transmitter 12 and the receiver station 18 may be close to each other or separated from each other. In either case, the antenna station 16 is located away from at least one of the receiver station 18 and the transmitter 12. Accordingly, in the following description, the transmitter 12 and / or the receiver station 18 may be referred to as “local” and the antenna station may be referred to as “remote”. Next, a feedthrough nulling system 19 is included, which includes a remote combiner 20 which is here a 180 ° directional coupler. , Antenna 24 provides a feedthrough zero residual signal on line 22 representing the power difference between the signal received at antenna station 16 and the feedthrough nulling signal on line 26. Downlink laser modulation transmitter 28 is located at antenna station 16 and is provided with a signal from downlink laser 29 at receiver station 18 and is provided with a zero residual signal on line 22 and on line 22. Results in an optical signal 30 representing the feedthrough zero residual signal. Downlink optical receiver 32 is located at receiver station 18 and detects optical signal 30 representing the feedthrough zero residual signal on line 22. Variable attenuator and phase shifter 34 includes a portion of the signal transmitted by transmitter 12, that is, a portion of the transmitted signal generated on line 36, and a detected optical signal representing the feedthrough zero residual signal at output 38. Is given. The variable attenuator and phase shifter 34 changes the gain and phase shift of the portion of the signal on the line 36 transmitted by the transmitter 12 according to the detected feedthrough zero signal at the output 38 to provide the appropriate gain and And a phase of the transmitted signal having a phase on the line 40 so that after an uplink optical transmission by the local uplink laser transmitter 42 to the antenna station 16, an appropriate feedthrough nulling signal is For the remote combiner 20 at the antenna station 16, it occurs on line 22.
[0011]
More particularly, the local uplink laser transmitter 42 is located at the receiver station 18 and has an appropriate gain to null the feedthrough signal 48 received at the antenna station 16. And an optical signal 44 representing a replica of the transmitted signal having a phase shift. Uplink receiver 50 is located at antenna station 16 and detects optical signal 44 produced by local uplink laser transmitter 42 to produce a feedthrough nulling signal on line 22.
[0012]
A noise generation system 52 is provided. The noise reduction system 52 includes a local detector (detector) 54, which in this example is located close to or adjacent to the local uplink laser transmitter 42 at the receiver station 18, and includes a local uplink laser transmitter. 42 detects a portion of the optical signal 44 produced and produces a noise cancellation signal on line 56. Here, the local noise cancellation combiner, which is the mixer unit 58, is provided with the noise cancellation signal on the line 56 and a part of the transmission signal on the line 36. The video signal generated at the output of the mixer unit 58 at the output 60 is combined with the video signal generated by the uplink receiver 32 in the addition network 58, and the gain / phase shift signal is sent to the variable gain / phase shift device 34. On output 64. The combining network 62 combines the output 60 of the noise cancellation combiner network 62 and the detected optical signal representing the feedthrough zero residual signal occurring on the output 38 of the downlink receiver 32, and the noise cancellation combiner 62. According to the detected zero residual signal on output 38 and output 60, the gain and phase shift of the signal on line 36 from transmitter 12 is changed. In this way, the replica of the transmitted signal on line 36 is adjusted in gain and phase so that after traveling from the uplink transmitter 42 to the uplink receiver 50 via line 40, the receiving antenna The feedthrough signal 48 received at 24 is nulled by the nulling signal on line 26 at 20 combined.
[0013]
Referring to FIG. 2, the radar system 10 is shown in more detail. The variable gain and phase shift device 34 includes a quadrature power divider (orthogonal power divider) 60 to which a portion of the signal transmitted by the transmitter section 12 is provided, as shown. The output of power divider 60 is thus a pair of quadrature signals (ie, in-phase (I) and quadrature (Q) signals). The in-phase and quadrature signals are provided to a power combiner 66 through a pair of variable attenuators 61 and 63 as shown. The output of the power combiner 66 is provided to the optical modulator 68 of the uplink laser transmitter 42 as shown. The modulator 68 is also provided with the output of the local uplink laser 70. Thus, the modulator 68 and the local uplink laser 70 constitute a local uplink laser transmitter 42. The output 44 of the modulator 68 is provided to a local diode detector (detector) 54 located in the receiver section 18 adjacent to the local uplink laser 70, as shown, and also to the antenna station. A remote uplink diode detector 72 located at 16 is also provided. The output of the remote uplink diode detector 72 is provided to the 180 ° directional coupler 20 via a transformer 74 as shown. Accordingly, the remote uplink diode detector 72 and transformer 74 provide a local uplink receiver 50 and a feedthrough nulling signal on line 26 as shown.
[0014]
The output of the receiver antenna 24 includes the feedthrough 32 from the transmitter antenna 45 and, as shown, through the optional amplifier 76, the second input of the 180 ° directional coupler 20. Given to. Thus, directional coupler 20 subtracts the energy received from receive antenna 24 from the feedthrough null signal on line 26 produced by uplink receiver 50 to produce a feedthrough zero residual signal on line 22. . The feedthrough zero residual signal on line 22 is provided to modulator 80 via bandpass filter 78 as shown. Also provided to the modulator 80 is the output 80 of the downlink laser 29 located at the receiver station 18. Accordingly, the bandpass filter 78, the modulator 80, and the signal 82 from the downlink laser 29 provide the downlink laser modulation transmitter 28.
[0015]
The output 30 of the remote downlink laser modulator transmitter 28 is provided to a diode detector (detector) 84. This diode detector 84 is included in the downlink receiver 32 as shown. The downlink receiver 32 includes a heterodyne unit 86 and an analog to digital (A / D) converter 88 to which the output of the diode detector 84 is provided, as shown. The output of A / D converter 88 is provided to a digital signal processor (DSP), for example, for conventional processing of the received signal. The output of the A / D converter 88 is also supplied to a quadrature (quadrature phase) mixer 90. Quadrature mixer 90 is also provided with a portion of the transmitted signal on line 36, as shown. The pair of quadrature output signals produced on line 38 by mixer 90 is integrated, ie, digitally filtered, and converted to an analog signal by digital-to-analog (D / A) converter 100, as shown in elements 96, 98. After being converted again, the control signal for the variable attenuators 63 and 61 in the variable gain and phase shift device 34 is provided to the low-pass filters 104 and 106 that produce the output 38 via the summing network 62 and the filters 108 and 110. I will provide a.
[0016]
In operation, a feedthrough nulling system 19 (FIG. 1) or loop is provided. Any residual feedthrough signal that occurs at the output 22 of the directional coupler 20 adjusts the gain and phase shift that the variable gain and phase shifter 34 generates in the portion of the transmitted signal on line 36, resulting in the appropriate A feedthrough zero signal is generated by the uplink laser transmitter 42. That is, the in-phase and quadrature signals in the I and Q channels of variable gain and phase shifter 34 are controlled by a control signal generated at output 64 in response to optically transmitted feedthrough zero signal 30 on line 22. This residual signal is brought close to zero.
[0017]
The output of the diode detector 54 is given to the quadrature mixer unit 58 as shown. A part of the transmission signal on the line 32 is also supplied to the orthogonal mixer unit 58. The output of the quadrature mixer unit 58 (i.e., the signals on the lines 92 and 94) passes through the integrators 96 and 98, the digital-to-analog converters 100 and 102, and the low-pass filters 104 and 106, and then in the summing network 62. In addition to the signal generated by the downlink receiver 32. With this arrangement, a local noise reduction loop 52 (FIG. 1) is obtained, while the feedthrough nulling loop 19 (FIG. 1) is simultaneously closed.
[0018]
More specifically, the local noise reduction loop 52 also has linear modulation capability. According to the feedthrough nulling loop 19, the control signal at output 38 (FIG. 1) linearly adjusts the transmitted signal samples on line 36, and on line 26 the feedthrough nulling signal is directed. Provided to the coupler 20. The closeout frequency of the feedthrough nulling loop 19 (that is, the frequency at which the open loop gain is equal to 0 db) is normally set to a low frequency, for example, smaller than 30 Hz. Since the feedthrough zero replica signal (the signal on line 26) must be provided to the coupler 20 via the optical uplink laser transmitter 42, the complexity of the uplink laser 70 (FIG. 2) itself being a noise source. Added. A typical Nd: YAG uplink laser has a noise level of -127 dBc / Hz at 100 Hz. The noise requirements for the optical uplink transmitter 42 are significantly more stringent than for the noise for the downlink laser 29. The reason is that the uplink signal-to-noise ratio is derived by the direct feedthrough level rather than by the residual signal after the feedthrough nulling addition in the directional coupler 20. As a result of the higher S / N ratio for the uplink laser 70, the noise of the uplink laser 70 will be approximately 30 dB better than the noise of the downlink laser 29. .
[0019]
In order to achieve this improved noise performance, the local noise reduction loop 52 (FIG. 1) is closed around the local uplink laser 52 (FIG. 1) located at the receiver station 18. Noise reduction loop 52 also linearly modulates uplink signal 44 using a feedthrough nulling control signal at output 38. A portion of the optical uplink signal 44 is recovered locally (at the receiver station 18) by the detector 54 and compared at a quadrature synchronous demodulator or mixer 58 with the signal from the transmitter, ie the signal on line 36. . The output 60 is the difference between the transmitted sample on line 36 and the signal recovered locally by the local detector 54 from the local uplink laser 42. The quadrature demodulated signal at output 60 is added to the feedthrough nulling control signal on output 38 in summing network 62, and what remains as a result at output 64 is loop integrator F (s) 108, 110. (Figure 2). Here, s is a Laplace operator. The feedback of the local noise reduction loop 52 results in a loop response that is equal to the feedthrough zero control loop 19 but opposite in polarity. The noise reduction loop 52 is closed via the variable gain and phase shift device 34 as shown in FIG. Here, the close-out frequency of the noise reduction loop 52 is 2 MHz, which guarantees significant noise reduction over the typical 100 KHz information bandwidth of the rf transmission waveform. Since the feedthrough zero loop 19 has a loop bandwidth of 30 Hz, the 2 MHz bandwidth of the noise reduction loop 52 is essentially transparent to the 30 Hz bandwidth of the feedthrough nulling loop 19 ( Transparent).
[0020]
As already mentioned, the optical uplink signal 44 is transmitted to two independent optical detectors 54, 72 (FIG. 1), a remote detector 72 and a local detector 54, here located at the receiver station 36. 2). In this configuration, the noise S / N ratio of the modulated uplink laser 70 is consistent with an error that is not greater than -30 dB. Thus, the local uplink laser 70 noise is detected and tracked by the local higher bandwidth noise reduction loop 52, and the local and remote detectors 54, 72 are operated by the feedthrough nulling loop 19. As matched, the noise generated by the uplink laser 70 is reduced at the remote detector 72. The receiver station 18, ie, the local high bandwidth noise reduction loop 52, reduces the noise of the local uplink laser 42 and linearly modulates the local uplink laser 42 using a feedthrough nulling signal. Acts like Therefore, when the local detector 54 and the remote detector 72 are matched, noise is reduced at the local detector 54 and also at the remote detector 72.
[0021]
Referring to FIG. 3, a radar system 10 is shown. In this figure, the variable gain and phase shift device 34, the uplink transmitter 42, the detector 54, the mixer unit 58 and the combining network described in FIGS. 1 and 2 are located at the transmitter station 12 ′, while Thus, the downlink laser 29 and the downlink receiver 32 are located at the receiver station 18 '. Note that, as in FIG. 1, the remote antenna station is remotely located from the receiver station 18 ', but the noise reduction loop 52 is located at the location of the local transmitter 12'. .
[0022]
Other embodiments are also included in the technical idea and scope of the appended claims.
[Brief description of the drawings]
FIG. 1 is a block diagram of a radar system according to the present invention.
FIG. 2 is a more detailed block diagram of the radar system of FIG.
FIG. 3 is a block diagram of a radar system according to another embodiment of the present invention.

Claims (4)

A radar transmitter that produces a transmission signal;
A radar receiver including a receiver station and a remotely located antenna station;
A feedthrough nulling system,
A remote combiner disposed at the antenna station and providing a feedthrough zero residual signal representing a power difference between a signal received by the antenna station and a feedthrough nulling signal;
A remote downlink modulator disposed at the antenna station to produce an optical signal representative of the feedthrough zero residual signal;
A downlink photodetector that is remotely located from the antenna station and detects the optical signal representative of the feedthrough zero residual signal;
Given a portion of the transmitted signal and the detected optical signal representing the feedthrough zero residual signal, and changing the gain and phase shift of the signal from the transmitter according to the detected optical signal; A variable attenuator and phase shift device that produces a replica of the transmitted signal having a gain and phase shift suitable for nulling a feedthrough signal received at the antenna station;
A local uplink laser modulator that produces an optical signal representative of the replica of the transmitted signal with a gain and phase shift appropriate to null a feedthrough signal received at the antenna station; and A feedthrough nulling system, comprising: an uplink detector disposed and detecting the optical signal produced by the uplink laser modulator to produce a feedthrough nulling signal;
Radar system with A local detector located remotely from the antenna station and detecting the optical signal produced by the uplink laser modulator to produce a noise cancellation signal;
A noise reduction system comprising a local noise cancellation combiner that is provided with the noise cancellation signal and the signal from the transmitter;
The variable gain / phase shift device combines the output of the noise cancellation combiner and the detected optical signal representing the feedthrough zero residual signal, and outputs the detected optical signal and the noise cancellation combiner. And changing the gain and phase shift of the signal from the transmitter to obtain a replica of the transmitted signal having a gain and phase shift suitable for nulling a feedthrough signal received at the receiving antenna station. The radar system of claim 1, which occurs. A radar transmitter;
A radar receiver having a receiver station and a remotely located antenna station;
A feedthrough nulling system,
A remote combiner disposed at the antenna station and providing a feedthrough zero residual signal representing a power difference between a signal received by the antenna station and a feedthrough nulling signal;
A downlink laser remotely located from the antenna station;
Located in the antenna station, given the downlink laser and the feedthrough zero residual signal, and modulating the light produced by the downlink laser with the feedthrough zero residual signal; A downlink modulator producing an optical signal representative of the feedthrough zero residual signal;
A downlink photodetector for detecting the optical signal representative of the feedthrough zero residual signal;
Given the signal from the transmitter and the detected optical signal representing the feedthrough zero residual signal, changing the gain and phase shift of the signal from the transmitter according to the detected optical signal A variable attenuator and phase shift device that produces a replica of the transmitted signal having a gain and phase shift suitable for nulling a feedthrough signal received at the antenna station;
Local uplink laser,
A gain suitable for nulling a feedthrough signal received at the receiving antenna station, provided with the uplink laser, the variable attenuator, and a phase-shifted gain and phase changed signal; A local uplink modulator producing an optical signal representative of the replica of the transmitted signal having a phase shift, and an antenna station, and detecting the optical signal provided from the uplink laser modulator A feedthrough nulling system comprising: an uplink detector that produces a feedthrough nulling signal;
Radar system with A local detector that detects the optical signal produced by the uplink laser modulator and produces a noise cancellation signal;
A local noise reduction system comprising a noise cancellation combiner that is provided with the noise cancellation signal and a signal from the transmitter;
The variable gain / phase shift device combines the output of the noise cancellation combiner and the detected optical signal representing the feedthrough zero residual signal, and outputs the detected optical signal and the noise cancellation combiner. And changing the gain and phase shift of the signal from the transmitter to obtain a replica of the transmitted signal having a gain and phase shift suitable for nulling a feedthrough signal received at the receiving antenna station. The radar system of claim 3 that occurs.

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